An astronomer has estimated that rocky, Earth-like planets may form around many, if not most, of the nearby Sun-like stars in our galaxy. His results suggest that worlds with potential as cradles for life might be more common than previously thought.

Using Nasa's Spitzer Space Telescope, Michael Meyer of the University of Arizona discovered that at least 20% and possibly up to 60% of stars similar to the Sun could potentially have rocky planets in orbit around them.

He presented his results at the annual meeting of the American Association for the Advancement of Science in Boston yesterday and they will be published in the February edition of Astrophysical Journal Letters.

His team looked at six groups of stars with masses comparable to the Sun. "We wanted to study the evolution of the gas and dust around stars similar to the Sun and compare the results with what we think the solar system looked like at earlier stages during its evolution," said Meyer.

Big balls of rock

Terrestrial planets are thought to form from giant clouds of dust that gather in a disc around stars. If these clouds become dense enough, they collapse in on themselves by gravity to form big balls of rock.

The Spitzer telescope was used to detect the infrared radiation emitted by the clouds of dust in orbit around nearby stars. The closer that dust is to the star, the hotter it will be, which allows astronomers to estimate how far out it is. Cold dust orbits far away from its star, whereas hot dust is probably right up close.

Habitable planets occupy the so-called "Goldilocks zone", at a distance from their parent star where it is neither too hot nor too cold. In practice this means a distance comparable to that between the Sun and the Earth or Jupiter.

Meyer found that around 10-20% of the stars had dust in their Goldilocks zones.

Live hard and die young

Another factor making it more likely that planets form in the correct places is that dust clouds of different sizes might form planets at different rates.

"An optimistic scenario would suggest that the biggest, most massive discs would undergo the runaway collision process first and assemble their planets quickly. Their discs live hard and die young, shining brightly early on, then fading," said Meyer.

Smaller discs, however, would take longer to form planets because there are fewer particles to collide with each other. If this idea is correct, then Meyer has calculated that as many as 62% of the surveyed stars have formed, or may be forming, planets.

"The correct answer probably lies somewhere between the pessimistic case of less than 20% and optimistic case of more than 60%," he said.